1. Field of the Invention
The present invention relates broadly to the art of electroless deposition of metallic films onto nonmetallic substrates. More particularly, the invention relates to a method for improving the color and durability of gold-coated articles.
2. Description of the Prior Art
In the art of depositing noble metal coatings onto nonmetallic surfaces, U.S. Pat. No. 3,300,328 to Luce discloses an aqueous electroless gold plating bath comprising a gold compound, an ammonium or alkali metal sulphite or meta-bisulphite complexing agent, and a hydrazine or hydroxylamine reducing agent. Gold films are deposited in about 40 minutes at elevated temperatures.
A more rapid method of depositing gold films onto nonmetallic substrates is described by Levy in U.S. Pat. No. 3,515,571. A preferably neutral gold solution is prepared by dissolving in water a gold salt such as gold chloride, and complexing the free gold ions in excess of 10.sup.-16 gram ions per liter with suitable coordinating ligands such as alkali metal carbonates, alkali metal hydroxides, ammonia and amines. Gold films may be deposited on nonmetallic substrates in about one minute at ambient temperatures by contacting a receptive surface with the above gold solution and a second solution of a hydrazine reducing agent. Levy suggests the use of the resultant gold coated articles as conductors, electrodes, and mirrors.
U.S. Pat. No. 3,484,263 to Kushihashi et al. discloses a method for forming a homogeneous semi-transparent gold coating on glass. The method involves contacting a sensitized glass surface with an alkaline aqueous solution of a gold salt, a reducing agent and an alkali carbonate to promote reduction at a temperature not to exceed 10.degree. C. After about 0.5 to 5 minutes contact, the contacting interface is subjected to radiation of 2500 - 5000 Angstroms to reduce the gold salt to a gold coating with a thickness of 150 - 500 Angstroms.
In U.S. Ser. No. 731,053, filed on Oct. 8, 1976, Franz et al., disclose an improved method for depositing uniform gold films by contacting a receptive nonmetallic substrate with a solution of complexed gold ions and a reducing agent. The improvement involves preparing the gold solution by adding a concentrated solution of a gold salt to a concentrated solution of a complexing agent with heating. A further improvement involves using sodium carbonate as the complexing agent and buffering the gold solution with sodium bicarbonate.